1. Field of the Invention
The present invention relates to a lumped filter having spiral inductors arranged in a circuit on a surface of a dielectric substrate, a shared antenna unit, and a communication device.
2. Description of the Related Art
A typical lumped filter is now described with reference to FIGS. 14A and 14B.
FIGS. 14A and 14B are a plan view and an equivalent circuit diagram of a lumped filter, respectively.
FIG. 14A illustrates an alumina substrate 1, an input terminal 2, an output terminal 3, a ground electrode 4, spiral inductors 5 and 6, interdigital capacitors 7 to 11, and LC resonators 12 and 13. A lower electrode, a dielectric film, and an upper electrode are laminated one on top of another on the alumina substrate 1. In FIG. 14A, the upper electrode is shown as being shaded and surrounded by solid lines, and the lower electrode is shown as being surrounded by broken lines.
FIG. 14B illustrates load conductances GA and GB, parallel inductances L1 and L2, parallel capacitances C1 and C2, an input capacitance C01, an output capacitance C23, and a serial capacitance C12.
The input terminal 2, the output terminal 3, and the ground electrode 4 are disposed on the dielectric film provided on the alumina substrate 1. The spiral inductor 5 having an upper electrode with a spiral configuration and the interdigital capacitor 7 are positioned side-by-side to define the LC resonator 12. The LC resonator 12 is connected between the input terminal 2 and the ground electrode 4. On the other hand, the LC resonator 13 defined by the spiral inductor 6 and the interdigital capacitor 8 is also connected between the output terminal 3 and the ground electrode 4.
The interdigital capacitor 10 is inserted between the input terminal 2 and the LC resonator 12, and the interdigital capacitor 11 is inserted between the output terminal 3 and the LC resonator 13. The interdigital capacitor 9 is inserted between the LC resonators 12 and 13.
In a circuit having such a structure, as shown in FIG. 14B, the LC resonator defined by the parallel inductance L1 and the parallel capacitances C1, and the LC resonator defined by the parallel inductance L2 and the parallel capacitance C2 are capacitively coupled through the capacitance C12. Thus, a lumped filter is provided and includes two resonators having the input capacitance C01 and the output capacitance C23.
However, such a typical lumped filter has the following problems which have yet to be solved.
The lumped filter shown in FIGS. 14A and 14B includes a number of circuit elements, thereby causing the line width and line spacing which are components of each circuit element to greatly affect the characteristics of the filter. These components are equivalent to the number of interdigitations, and the line width forming an interdigitation, and the line spacing in interdigital capacitors, and equivalent to the number of turns, the line width, and the line spacing in spiral inductors. It is therefore more difficult to design the plurality of components within predetermined tolerances as the number of components increases, thereby keeping a low yield of the filter including such elements.
While predetermined transmission characteristics are achieved, in order to further increase the Q factor of the filter, a way to provide an attenuation pole at a high-frequency region or a low-frequency region of the pass-band can be contemplated. However, the typical lumped filter cannot have an attenuation pole unless a new circuit element is provided.
In order to overcome the problems described above, preferred embodiments of the present invention provide a lumped filter having a simple structure with a high band-pass characteristic, a shared antenna unit, and a communication device incorporating such a lumped filter and the antenna unit.
According to a preferred embodiment of the present invention, a lumped filter includes a dielectric ceramic substrate including a plurality of inductors, a plurality of capacitors, an input terminal, an output terminal, and a ground electrode, which are arranged to define a circuit on the dielectric ceramic substrate. A plurality of parallel elements connected between the input terminal and the ground electrode, and between the output terminal and the ground electrode include parallel LC resonators each being defined by the inductance of a spiral electrode and the capacitance produced between the spiral electrode and an adjacent electrode. A plurality of series elements inserted between the parallel elements include parallel LC resonators each being defined by the inductance of a spiral electrode and the capacitance produced between the spiral electrode and an adjacent electrode. Therefore, the lumped filter can be simple and have a high pass-band characteristic.
The number of turns, the line width and line spacing in the spiral electrode of the series element may be set at predetermined values to determine the frequency of an attenuation pole produced by the self-resonance of the parallel LC resonator. This allows an attenuation pole to be produced at a desired position, thereby providing a desired pass-band characteristic for the lumped filter.
The number of turns in the spiral electrode of the series element is preferably greater than the number of turns in the spiral electrodes of the two parallel elements adjacent to the series element. The lumped filter can thus have a simple construction while achieving a high pass-band characteristic.
At least one of the series elements inserted between the input terminal and the parallel element, and between the output terminal and the parallel element may include a capacitor defined by a lower electrode, a dielectric film, and an upper electrode disposed one after another on the dielectric ceramic substrate. Therefore, the lumped filter can be simple.
The spiral electrode may be a thin film having superconductivity. With this structure, the lumped filter can have a significantly reduced insertion loss in an environment of the transition temperature or lower. Because of substantially no conductor loss of the electrodes, the lumped filter can be compact.
In another preferred embodiment of the present invention, a shared antenna unit includes the lumped filter according to the preferred embodiment described above. Therefore, a shared antenna unit having a high attenuation characteristic in which the transmission side and the reception side provide a sufficient amount of attenuation in the mutual pass-bands can thus be constructed with simplicity.
In another preferred embodiment of the present invention, a communication device includes the lumped filter or the shared antenna unit according to the preferred embodiments described above. Therefore, a communication device having a high communication characteristic can thus be constructed with simplicity.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.